The present invention relates to magnetoresistive devices. More specifically, the present invention relates to using a magnetoresistive element as a sensor in systems such as data storage systems.
A magnetoresistive (MR) element exhibits a change in electrical resistance as a function of external magnetic field. This property allows MR elements to be used as magnetic field sensors, read heads in magnetic storage systems, and magnetic random-access-memories. Depending on the structure of a device, the MR effect can fall into different categories, namely, anisotropic MR (AMR), giant MR (GMR), tunneling MR (TMR), and colossal MR (CMR). The early magnetoresistive devices, many currently still in production, utilize an AMR sensor. However, since an AMR sensor typically uses an AMR effect film such as NiFe, its magnetoresistive ratio and sensitivity are low. More recent magnetoresistive devices take advantage of the phenomena of GMR, TMR or CMR, which have come to light in recent years and attracted much research effort because these phenomena afford a greater MR ratio compared to the AMR effect.
While the AMR effect is generally due to an inherent magnetoresistive effect of a homogeneous material such as Fe or NiFe, the GMR, TMR and to a certain degree CMR effects are made possible by a spatial arrangement of several different materials. The GMR effect, for example, is achieved by utilizing a spin valve with a multilayer structure constituted of a nonmagnetic metallic layer sandwiched between two ferromagnetic layers. Such spin valves demonstrate characteristics of a high magnetoresistive ratio and high sensitivity with respect to the strength Oman external magnetic field. The GMR effect in such a structure is associated with a change in the relative alignment of the net spins on two ferromagnetic layers. When the net electron spins (or magnetization) on the adjacent ferromagnetic layers are in opposite directions, the resistance is high. When they are in the same direction, the resistance is low. Although an exact quantitative explanation for the GMR effect requires ab initio quantum mechanics calculations, a quite simple qualitative explanation is available. The electrical resistance of a metal arises from irregularities and discontinuities in the atomic lattice potential, called defects, as seen by the electrons. The defects cause electrical resistance by scattering electrons carrying a current. In a normal homogeneous conductive material such as a metal, both up-spin electrons and down-spin electrons travel in their own smooth lattice potential, resulting in low electrical resistance and high electrical conductance. In comparison, in the sandwiched multilayer structure previously described, the “smoothness” of the lattice potential seen by either up-spin electrons or down-spin electrons is disrupted by the nonmagnetic metallic layer in the middle because of a mismatch of the state densities of one of the spin polarizations between the nonmagnetic layer and the ferromagnetic layers. This can be illustrated in an exemplary case in which the ferromagnetic layers are cobalt and the nonmagnetic layer is copper. When spin polarizations in the two cobalt layers have the same direction, the up-spin electrons notice little difference in the number of electrons per atom as they travel from the cobalt layer to the copper layer. To these electrons the lattice potential is smooth or defect free. The electrical resistivity in this aligned case is thus low because up-spin electrons experience very little resistance and act like a short circuit, making the electrical resistance experienced by the down-spin electrons relatively irrelevant. When spin polarizations in the two ferromagnetic layers have opposite direction, however, both the up- and down-spin electrons experience an interrupted lattice potential at one of the interfaces because of a mismatch of the densities of electronic states between cobalt and copper, thus giving rise to a higher electrical resistance.
TMR is a generally similar phenomenon. The most elementary TMR system is obtained simply by replacing the metallic nonmagnetic spacer layer of a GMR sandwich configuration by an insulating layer (often called barrier layer). Unlike the GMR effect in which the spin polarization dependent magnetoresistance has to do with the conductive property of the middle layer, the TMR effect is caused by quantum tunneling through a very thin barrier layer which is an insulating material if observed as a bulk material. In the TMR phenomena, a tunneling magnetoresistive effect manifests depending upon the relative angles of magnetization of two ferromagnetic layers on two sides of a nonmagnetic insulating barrier layer in a multilayer junction similar to the sandwiched structure in GMR. The tunneling magnetoresistive effect is believed to be a result of the asymmetry in the density of states of the majority and minority energy bands in the ferromagnetic material. The resistance, which is inversely proportional to the spin-polarized tunneling probability, depends on the relative magnetization orientations of the two ferromagnetic layers on either side of the insulating barrier layer. In the parallel orientation there is a maximum match between the number of occupied states in one ferromagnetic layer and that of the available states in the other, resulting in a relatively higher tunneling probability. In the antiparallel configuration, the tunneling is between the majority states in one ferromagnetic layer and minority states in the other. This mismatch of the density states diminishes the tunneling probability. TMR often has a higher magnetoresistance ratio (signal ratio) than that of GMR, but more importantly an optimized TMR structure has proven to be able to provide extremely high degree of magnetic field sensitivity for small magnetic fields.
Both GMR and TMR have found important applications in various industries, primarily utilizing their ability to recognize an external magnetic field or a change thereof. A noted example of such applications is found in read heads of computer hard disc drives. However, the applications are by no means limited to computer hard disc drives. For example, using GMR or TMR, it is possible to make computer operating memories, such as random access memory (RAM), that are immune to power disruptions and ionizing radiation. GMR or TMR may also be used in motion sensors to increase the efficiency and safety of home appliances, automobiles, and factories. In principle, any motion that causes a change of the strength of a magnetic field can be detected by a sensor based on either GMR or TMR. Applications of this type of devices therefore may be widely used in the industrial, commercial, and military fields. Possible applications include sensitive detectors for wheel-shaft speed such as those employed in machine-speed controllers, automotive antilock brakes, and auto-traction systems; motion and position sensors for electrical safety devices; current transformers or sensors for measuring direct and alternating current, power, and phase; metal detectors and other security devices; magnetic switches in appliance controls, intrusion alarms, and proximity detectors; motor-flux monitors; level controllers; magnetic-stripe, ink, and tag readers; magnetic accelerometers and vibration probes; automotive engine control systems; highway traffic monitors; industrial counters; equipment interlocks; and many other applications requiring small, low-power, fast sensors of magnetic fields and flux changes. Furthermore, suitable film-deposition processes may also permit fabrication of GMR or TMR devices on electronic-circuit chips to produce highly integrated sensors at low cost and high volumes for mass industrial markets. Furthermore, magnetoelectronic devices such as field effect transistors (FETs) may be developed based on the magnetoresistive effect and these devices may someday complement or even replace some semiconductor electronic devices.
In storage systems, such as computer hard disc drives, the read head uses a magnetoresistive element. The read head is typically merged with a writer head. The writer writes encoded information to the magnetic storage medium, which is usually a disc coated with hard magnetic films. In a read mode, a magnetic bit on the disc modulates the resistance of the MR element as the bit passes below the read head. As drive storage areal density increases, GMR or TMR read sensors using a magnetoresistive element become increasingly important. The digital information (bits of 1 or 0) is stored as the direction of the magnetization of small regions on the disc. The information is read by sensing the magnetic fields just above these magnetized regions on the disc. As the areal density becomes higher, the regions become smaller, and the fields that must be sensed to read the data become weaker. Read sensors that employ the GMR or TMR effect provide the best technology currently available for detecting the fields from these tiny regions of magnetization. These very small sensors detect a very small magnetic field that causes a detectable change in resistivity in the sensor due to the magnetoresistive effect. The detected changes in the resistivity produce electrical signals corresponding to the data on the disc. The electric signals are then sent to the computer to be processed.
The GMR element favored by the data storage industry is the spin valve. It consists of a free ferromagnetic layer which rotates with the external field, a conductive spacer, and a pinned ferromagnetic layer which has a magnetization fixed along one direction. The electrical resistance of a spin valve is a function of the angle between the magnetization in the free layer and the pinned layer. A GMR sensor is the most resistive when the two layers are magnetized in anti-parallel directions, and is the most conductive when they are parallel. Most hard disc makers have completed the transition from making AMR heads to making GMR heads. The technology may work for areal densities up to 100 G bit/inch2, beyond which point the sensitivity again becomes an issue.
TMR devices offer a possible solution to achieve even higher areal densities. Compared to GMR devices, TMR devices usually have greater output signals and are also more sensitive to small external magnetic fields. TMR read heads in computer hard disc drives have been disclosed, for example, in U.S. Pat. No. 5,390,061; and U.S. Pat. Nos. 5,729,410, 5,898,547, 5,898,548, and 5,901,018.
A common TMR element used in read heads of hard disc drives is a TMR junction very similar to a spin valve in the sense that it also consists of a free layer, a middle layer, and a pinned layer (often called reference layer). The magnetoresistance arises from the angular difference between the magnetization in the two magnetic layers in a way analogous to a spin valve. A major difference between a TMR junction and a GMR based spin valve is that the middle layer in a TMR junction, commonly called a barrier layer, is made Oman insulator, typically aluminum oxide, instead of a conductor. Moreover, in typical TMR sensors the electrical current is perpendicular to the plane (CPP) of the films as opposed to current in the plane (CIP) for GMR sensors. Consequently, TMR junctions require a top and a bottom electrode to the junction stack in order to measure the electrical property.
A GMR element and a TMR element as described above both use a middle layer sandwiched between two magnetic layers. The middle layer used in these conventional magnetoresistive elements is called “spacer” when the element is based on the GMR effect and “barrier” when the element is based on the TMR effect. Conventionally, the engineering designs for GMR based magnetoresistive elements and TMR based magnetoresistive elements took different approaches as far as the selection of the material and structural designs for the middle layer (spacer or barrier, respectively) is concerned. Specifically, metallic materials are used for spacers, while insulators are used for barriers. In either case, however, inorganic material has been used for the middle layer (spacer or barrier). Although organic materials have been suggested for making an active element in a magnetoelectronic device to transport spin-polarized electrons (see U.S. Pat. No. 6,325,914), there has been no suggestion to use an organic material to make a passive spacer or barrier in a GMR or TMR element. An active element used to transport electrons in a magnetoelectronic device is a relatively thick material, requiring a thickness greater than 50 nm (500 Å) to ensure the electrical continuity of the material. In contrast, a passive spacer or barrier in a GMR or TMR element generally has a thickness less than 5 nm (50 Å).
Due to the importance of a basic magnetoresistive element used in various magnetoresistance devices, there is a need for new designs for such a magnetoresistive element that is smaller, more manageable in fabrication, and exhibits more reliable, more predictable and more sensitive magnetoresistive effect. Factors that contribute to the above characteristics include the chemical properties, the physical properties and the thickness of the barrier layer. For example, the use of a TMR element in computer hard disc read heads may allow for increased magnetoresistance signal for high areal density heads, but the application is limited in part by the characteristics of high resistance of the barrier layers. The prior attempts to minimize the resistance of the barrier layers typically include reducing barrier layer thickness. The existing read sensor designs have included insulating barrier layers formed of insulating oxides such as alumina. Alumina insulating barrier layers can be formed by known methods, including deposition of aluminum metal by physical vapor deposition, evaporation, or ion beam deposition. After such deposition, the aluminum can then be oxidized in O2 plasma or by simple, controlled exposure to O2. Such processes can result in an alumina layer having a thickness T in the range of about 10 Å to about 50 Å, and an effective tunneling barrier for electrons in the range of about 1 eV to about 5 eV. Empirically, a lower effective tunneling barrier for electrons is at least partially due to a lower band gap of the barrier layer material used, where the band gap is a measure of the separation between the energy of the lowest conduction band and the highest valence band. While such thicknesses and band gap values may be adequate, unfortunately these processes and materials result in significant defects such as pinholes, and therefore significant probability of shorting. Using conventional materials and conventional methods, the thinner the barrier layer becomes, the more likely the resultant barrier layer contains pinholes and other defects.
At the same time, existing barrier layer configurations are limited by the minimum thickness possible without suffering magnetic coupling between reference and free layers. Specifically, the thinner the barrier layer is, the closer the reference and free layers are to each other, and the more likely magnetic coupling between the reference and free layers becomes. Severe coupling may render the read head ineffective for detecting external magnetic field signals.
Inorganic barrier layers having a band gap smaller than that of alumina have been proposed as a means of reducing the barrier layer's electrical resistance. U.S. Pat. No. 6,330,137 to Knapp et al. discloses a magnetoresistive device that includes an insulating barrier layer formed of diamond like carbon (DLC) such as tetrahedral amorphous carbon. The use of tetrahedral amorphous carbon is said to be more defect free than other prior art insulation barrier layers while maintaining at least comparable band gap values and thicknesses. But such applications are limited to amorphous carbon materials, and the number of such materials is limited.
Furthermore, as the thickness of the barrier layer reaches within the range of nanometers, the physical and chemical properties of the barrier layer can no longer be predicted based on its bulk material properties. This ostensible obstacle is actually a blessing that leads to a new dimension of engineering designs at the molecular level. In this regard, conventional materials used in TMR as a conventional barrier layer has limited amount of manipulability and limits the freedom for engineering designs at the molecular level.